Spinning of polyacrylonitrile filaments



March 17, 1959 F, HALBlG 2,878,097

SPINNING OF' POLYACRYLONITRILE FILAMENTS Filed Aug. 420, 1957 ad (1f/6% m@ @/a" BY w; www4 SPINNING OF POLYACRYLONITRILE FILAMEN TS Paul Halbig, Fribourg, Switzerland Application August'20, 1957, Serial No. 679,244

3 Claims. (Cl. 18`54) f The invention relates to the spinning of polyacrylamitrile fibers.

In my copending application, Serial No. 311,530, led September 25, 1952, which is a continuation in-part of my application SerialNo. 236,081, tiled July 10, 1951, now abandoned, I have disclosed'a method for preparing nitric `acid solutions of polyacrylonitrile, which are stable at temperatures below 20 C. for many days without hydrolysis of the nitrile group.

Said method consists in dissolving polyacrylonitrile prepared by polymerization of acrylonitrile in' the presence of anorganic peroxide, for instance acetyl peroxide, as catalyst, at temperatures of to 80 C. in nitric acid having a concentration of 49 to 68% HNOS. Preferably, the polyacrylonitrile is rst suspended in a 46-54% HN03 at temperatures of 0-20 C., and degassed and subsequently dissolved at said temperature by adding higher concentrated nitric acid until the HNOgconcentrationof the solution is about 59%. f

Under the recited conditions, the nitric acid is, like dimethylformamide, a true solvent for polyacrylouitrile; incomparison therewith, it is cheaper and has the advantageiof allowing processing of the polyacrylonitrile without discoloration.

The nitric acid solutions of polyacrylonitrile aresuitable for wetspinning; in such wet spinning procedures, the nitric acid polyacrylonitrile solution is extruded at low States Patent() i temperature into aqueous nitric acid andthe vformed laments are washed with nitric acid of gradually decreasing concentration, and finally with water. Subsequently, they are stretched, shrunk, and dried to obtain a ber suitable fortextilepurposes. By washing-in steps, almost the total amount ofthe nitric acid introduced as solvent may be recovered in relatively concentrated form and can be conf centrated by removal of water to HNO3 content required for dissolving the polyacrylonitrile. When using a 40% HNO3 .solution as spinning bath, the nitric acid used as solvent is readily recovered in the form of an about 40% acid.

Thespinning of unhydrolyzed solutions into a precipitating bath containing more than 30% of HNOS presents certain diiliculties which are avoided when spinning solutions are used in which part of the polyacrylonitrile has been hydrolyzed to the amide. On the other hand, the useof such partially hydrolyzed solutions-has the drawback that it is diicult to maintain a uniform hydrolysis degree; fabrics made of polyacrylonitrile bers of differing hydrolysis degree show uneven behavior in processing, particularly withrespect to dye absorption.- A particular drawback of partially hydrolyzed polyacrylonitrile bers is their reduced weather resistance.

. In `view of the recited drawbacks of partially hydro` lyzed polyacrylonitrile bers, it is a principal object of Patented Mar. 1.7, 1959 2 this invention to provide a method of spinning unhydrolyzed nitric acid polyacrylonitrile solutions.

Other methods and advantages will be apparent from a consideration of the specication and claims. Y g .y

The diliiculties encountered in spinning unhydrolyzed solutions of polyacrylonitrile in nitric acid are connected with the HNO3 concentration of the spinning bath. If the spinning bath has a concentration of 30% HNO3, the spin-y ningof satisfactory fibers is possible only at temperatures around 0 C. If thevHNOa concentration in thespinniug bath is raised to about 40%, the temperature ofthebath must be decreased below 0 C. At such low temperatures, the spinning solution tends to gel, which results in further diiiiculties. The principal diiculty for a technical procedure, however, is due to the fact that the ratel of drawing of the lament from the spinning nozzle decreases with increasing concentration of the HNOa in the spinning bath. While it is possible to spink into a coagulating bath containing less than 40% of HNO3l at a rate of feed which is close to the theoretical rate, coagulating baths containing 40% of HNOa and more,lower rates of drawing are required. This limitation becomes particularly troublesome if attempts'a-re made to increase the yield ofthe spinneret, as shown in Table I below.

In said table, V1 designatesthe'theoreticalrate of'de livery, in m./min., as calculated from the amount of solution Q (cc./min.) fed by .the spinning pump to the spinneret, the number Z'of the spinneret'holesaud the diameter thereof in mm., according to the equation The gures of Table I were obtained with a 10% solution of polyacrylonitrile in 59% HNO3 and a'coagulating bath of 40-43% HNO3 at a bath temperature of 4`to -7 C. The spinneret was made of chromium-nickel steel and had 10 holes of .2 mm. diameter each. V2 desi ignates the maximum rate of drawing at'whichope'ration was still possible. G defines the rate of drawing inpercent of the rate of delivery V1. l TABLE I [Coaguitmg bathliosafz, HNOS; -4 to 7 0.] i

Qv v1, m. vtm. i G, per;

' i CB1-1t I have'foundv that V2 canbe increased abruptlyv many times, when, under otherwise unchanged conditions, the concentration of the coagulating bath is increased `to Llll-46% HNO3. This is shown in Table Il. j

TABLE II [ooaguianng bath 4440175, HNo; -4 to 7v 0.]

Q Vi', m. V2, In; G, percent ,The Atablesl show that HNO3 content below 43% HNO3 allow a maximum rate of drawing only below 100% of the rat'e of delivery, While on increase of the HNO3 content in the coagulating bath to above 43%, the rate of drawing can be suddenly increased to several 100%. If the HNO3 concentration in the coagulating bath exceeds 46%, the spinning operation stopsbe,cause the spinning solution leaving the spinneret coagulates too slowly. The Vincrease of the rate of drawing is possible only within the narrow limits of 43-46%, preferably 4511 HNOa, in the coagulatin-g bath. I do not have a satisfactory explanation for this phenomenon, but I believe that there may be some kind ofconnection with the observation that polyacrylonitrile is not soluble below 100 C. in nitric acid weaker than 46% (see Swiss Patent No. 299,374).

With respect to Table II, it may be noted that it is, of course, also possible to spin at a rate of drawing below the maximum rate, for instance aty a rate of 150- 300%. However, such ilaments have, after processing, a hazy and opalescent appearance. This drawbackcan be obviated by decreasing the temperature of the coaguaereos? coagulating baths having a lating bath, but this increases, as set forth above, the

in other methods of spinning polyacrylonitrile filaments from nitric acid solutions, where theobtained laments could be stretched only after washing at elevated temg peratures. Such stretching has not the same favorable effect on the filaments as the cold stretching in the coagulating bath. The stretchediilaments obtained according y to the invention in the coagulating bath can be, of course, after-stretched after washing, and processed like other polyacrylonitrile ber to impart thereto specic properties.

The method of the invention may be carried out as follows: The spinning solution is forced by a metering pump through the spinneret submerged in the coagulating bath. The required pressure is, even at high rates of spinning, relatively low, for instance 2-10 kg./cm.2, since even very line titers may be spun with relatively large holes of the spinneret: forinstance, individual titers of .2 denier can be obtained with holes of .l0 mm. diameter. The coagulation [and solidication of the spinning solution entering the cold coagulating bath proceeds very of HNO3.

` solutions may be kept constant by passing wash water into the fourth bath and transferring solution from one wash step to the next higher concentrated bath. 43- 46% nitric acid is drawn olf from the coagulating bath, which nitric acid is concentrated by fractionation and then used again as solvent for the polyacrylonitrile. After complete removal of the nitric acid from the filamentary bundles, the bundles can be processed in the usual manner, for instance by stretching them at high temperatures, shrinking and drying.

The following examples are given to illustrate the method of the invention with reference to the attached flow-sheet indicating the sequence of the various steps. All parts and concentrations are given by weight, unless indicated otherwise.

Example 150 g. of finely powdered polyacrylonitrile having an average molecular weight of 60,000, a density of .4, and a particle size of .0l-.10 mm., are stirred at 0 C. into 525 g. of aqueous nitric acid containing 54% of HNO3. To the thus obtained suspension, 855 g. of aqueous nitric acid containing 63% of HNO3 are added at an absolute pressure of 20-40 mm. Hg. By stirring at 0 C., the mass is converted to a clear solution within a few minutes. Said solution contains 9.8% of polymer in aqueous nitric acid of 59.5% HNO3. The gelling temperature of the solution is at about -9 to 12 C. The solution can be stored at temperatures of 0 to 5 C. for at least 250 hours without chemical changes. The viscosity of the solution at 15 C. is 90 P.

The solution is filtered at 0 C. through a filter material consisting of polyvinyl chloride, degassed and forced by a metering pump of acid resistant steel through a spinneret submerged in the coagulating bath, which consists of aqueous 44-45% nitric acid. The temperature of the coagulating bath is adjusted to -5 to 7 C. The obtained laments are taken up as tow by a drawing device rotating in 'the spin bath and passed therefrom to a second drawing device which rotates in a bath of aqueous nitric acid of about 0 C. containing about 16% Both drawing devices consist of a godet wheel which is capable of taking up and giving olf the tow. From the drawing device, the tow is passed into a washing device, Where it is washed acid-free with water at room temperature. Subsequently, the lamentary bundle is passed through a boiling water bath (stretch quickly. Already a few centimeters behind the spinneret, the separate filaments do no longer stick together and may be combined to a bundle which is passed through a first drawing device. Subsequently, the tlamentary .bundle is Washed stepwise with gradually weaker nitric acid, and finally with water. The rate of drawing is adjusted to about 400% of the theoretical rate of delivery from the spinneret, which results in a cold stretching of the tlaments. The mechanical devices used may be the same as conventionally used in the wet spinning of viscose,

f (a) The number of the example; y

The coagulating bath is maintained at a temperature of -4 to 7 C. and at a HNO3 concentration of'43- 46%, preferably 45 il%. The temperature of the washing solutions of lower HNO3 concentration may be higher by a few degrees, and the temperature of the wash water may go up to Ztl-30 C. The concentration of the i 4 washing steps, where the first bath contains 4346%, the2 second bath 15-20%, the third bath 4-6% and the fourth bath .5 to V2% of HNOa. The concentration of said bath bath) to a third drawing device, and hence it is passed through a second boiling water bath (shrink bath) and drier, which is operated with air of 60-90 C., to the end bobbin, where it is wound up with a tension'of less than 10 g. per 100 denier.

The seven examples of Table III were carried out in the above described apparatus. The columns of the Table III designate (b) The amount of solution fed by the metering pump'v to the spinneret in cc./min.; l v (c) Number and diameter of the spinneret holes, (d) Theoretie rate of delivery V1 in m./min., calculated from the amount of solution, number and diameter of spinneret holes (see equation in column 2, line 30); (e')(i) Rates of drawing at the five drawing devices. The invention is concerned only withthe ratio of the rate of drawing IV, listed in column g, to the rate of delivery V1 given in column d; (k) Stretch of the fresh filament in aqueous nitric acid between the admission of the solution through the spinneret (column d) and the washing device (column g) as percentage of the theoretic rate of delivery V5" (l) `Titer of the finished individual lament in denier;

(A, 4 Tensile strength of the individual filament in g./den.; (it) Elcngation at break in percent.

TABLE 111 (a) f '(6) (c) (d) (e) (I) (0) (b) (i) (k) (l) l (1n) (n) l Tensile Elonga- N o. cc./min. Spmneret I II III IV V VI Stretch m Titer, strength, tion at.

IIL/mm. m./mln. `m./min. m./mln. m./mm. m./min. percent Denier g./Denler break,

` percent 2. 6 100/0. l5 1. 48 1. 8 5. 8 6. 0 12 10 405 3. 1 3. 1 22 2. 6 100/0. 30 0. 36 1. 8 5, 8 6. 0 12 10, 1, 670 3. 1 4. 1. A 18 2. 6 100/0. 15 1. 48 3.0 11. 8' 12.0 42 v 34 810 40. 9 4. 2 1-7 12. 100/0. 15 6. 80 12.0 35. 5 36. 0 100 80 530 1. 8 2.9 21 2. 6 400/0. 10 0. 82 1. 3 l. 3 3. 9 12 10 475 0. 77 3. 7 23 1. 48 400/0. 10 0. 47 1. 3 1. 3 7. 0 24 20 1, 490 0. 22 3. 2 25 16. 7 1, 000/0. 0. 53 1. 8 1. 8 6. 0 51 43 1, 130 0. 46 4. 2 15 In Example 1, a spinneret with holes of .15 mm. diameter was used. Example 2 was carried out in the same manner as Example 1 but with a 100 holes spinneret of .3 mm. size of holes. In both cases, faultless laments were readily obtained, though for the 0.15 mm. hole size spinneret the rate of delivery was four times the rate of delivery for the .30 mm. spinneret. The stretch between spinneret and washing was 400 and 1600%, respectively.

A comparison of Examples l, 2, and 3 shows -thergr'e'at elasticity of the process, since with the same apparatus deniers of individual filaments between .9 and 3.1 denier are spun.

Example 4 illustrates the operation with high spinning output.

According to Examples 5 and 6, individual deniers of .77 and .22, respectively, were spun in the same apparatus with a spinneret of 400 holes of size .1 mm. The possibility to spin such fine deniers illustrates the great possibilities of the new process.

Example 7 shows spinning with a 1000 hole spinneret.

None of the examples implies any limitations of my method. A spinneret with .4 mm. holes allows of spinning the same denier under the same conditions as .15 mm. holes, though the difference in the rate of delivery is 700%.

The cold stretchability is greatest as long as the filamentary structures are still impregnated with the acid of the coagulating bath. In the first washing step with dilute nitric acid, the stretchability may be still more than 600%. Then it decreases, and after complete removal of the nitric acid the stretchability increases again only at elevated temperatures.

According to the invention, the coagulation and wet stretching takes place under acid conditions about at room temperature and below, for instance at +30 to -10 C., preferably at 5 to 8 C. Subsequently, heat-stretch may be applied. The total stretch, that is: the wet cold stretching between spinneret and washer under acid conditions, plus the conventional heat-stretching, may attain the huge amount of 10,000 percent of the rate of delivery at the spinneret; this means that it is possible to spin with relatively large-holed spinnerets fast very line filaments.

The filaments obtained according to the method of .the invention are white and silky; they have high tenacity and elongation, and good weather resistance. In a comparative test filaments obtained by my novel method were exposed for 14 months to the atmosphere at Fribourg (640 m. above sea level; 46 northern latitude) on a field 30 cm. above ground. At the end of said period, the loss of strength was only percent of the initial strength. Filaments which had been prepared from the same polymer but had been dissolved in nitric acid with hydrolysis of 20 percent of the nitrile groups and then spun, lost 90 percent of the initial strength under` the same conditions.

The invention overcomes the difficulties which heretofore discouraged the use of nitric acid solutions of polyacrylonitrile for spinning operations. The advantages may be summed up as follows:

(1) By the use of nitric acid as solvent, polyacrylonitrile can be converted quickly and very economically into filaments. The solution can be accomplished within a short time at low temperature; ageing or heating of the very viscous spinning solution is not necessary. The solution is fed to the spinneret at the same temperature at which it was obtained. There is no time consuming and complicated heating operation, which is, for instance, required for the preparation of spinning solutions with dimethyl formamide. By means of nitric acid, the polymer powder can be converted within a few hours to filaments of the same chemical composition as the original polymer. In this operation, a temperature of 100 C. is not exceeded, and the pure color of the polymer powder is therefore retained. As the various manipulations do not require much time, the whole plant may be relatively small. There are no corrosion problems because, wherever nitric acid is present, temperatures of 30 C. are not exceeded. A large part of the apparatus can be built from cheap synthetics such as polyvinyl chloride. In view of the low temperatures, no health problems arise with respect to the protection of the operators.

(2) The concentration of the nitric acid withdrawn from the coagulating bath is about 45% HNO3 and therefore quite close to the concentrations of 54 and 63% required for the solution of the polyacrylonitrile. Therefore, the solvent is readily regenerated by distilling off some water.

(3) The high stretchability in the acid state allows the fast and safe spinning of relatively fine filaments with relatively large holes of the spinneret, that is under relatively low pressure.

(4) The conversion of the polymer into filaments consumes only water and energy, since the nitric acid can be substantially recovered by simple and economic procedures. In addition, nitric acid is cheap in comparison with the organic solvents conventionally used for spinning polyacrylonitrile, and small losses thereof are of little importance.

The discovery that nitric acid in the narrow range of 44-46% HNOS is an excellent coagulating bath for nitric acid spinning solutions of polyacrylonitrile, opens the way for the commercial and economic manufacture of polyacrylonitrile filaments from such solutions.

The term polyacrylonitrile as used in the specification and claims is intended to designate also polymers which contain at least percent of polyacrylonitrile, the balance being an olefinic compound copolymerizable with acrylonitrile.

Iclaim:

1. A method of preparing polyacrylonitrile filaments comprising extruding a substantially unhydrolyzed spinning solution of polyacrylonitrile in 56 to 65% HNOS through a spinneret into a coagulation bath consisting essentially of aqueous 43-46% HNO3 and having a ternperature of about +30 to 10 C. passing the formed filaments successively through washing zones containing sequentially dilute nitric acid and water, and drawing the freshly formed filaments on their travel from the spinneret to said water washing zone at a rate of at least i7 '8 400 percent of the rate of extrusion of said solution.. the laments is compensated by transfer of weaker nitric through said spinneret. `a"c`id wash solution from the succeeding Washing step.

2. The method as dened in claim l, wherein the la ments are Washed rst in nitric acid of about 15 to 20 'References Cited m the me of this patent percent HNO3, thenin nitric acid of about 4 to 6 per- 5 UNI TED STATES PATENTS Cem HNO and n'ally in nitric @wild 0f 510. 2. Percent, #2,577,763` .Home Dec1'11,19s1

HNOa- 2,579,451 Palson Dec.18, 1951 1 3. The method as dened 1n claim 2, wherein the m- FOREIGN PATENS' A` crease of the HNOa concentration of said wash solution Y Y A by the nitric acid of the spinning solution adhering to 10 744,278 Great Britain `Feb; 1,'19$6 

1. A METHOD OF PREPARING POLYACRYLONITRILE FILAMENTS COMPRISING EXTRUDING A SUBSTANTIALLY UNHYDROLYZED SPINNING SOLUTION OF POLYACRYLONITRILE IN 56 TO 65% HNO3 THROUGH A SPINNERET INTO A COAGULATION BATH CONSISTING ESSENTIALLY OF AQUEOUS 43-46% HNO3 AND HAVING A TEMPERATURE OF ABOUT +30 TO -10*C. PASSING THE FORMED FILAMENTS SUCCESSIVELY THROUGH WASHING ZONES CONTAINING SEQUENTIALLY DILUTE NITRIC ACID AND WATER, AND DRAWING THE FRESHLY FORMED FILAMENTS ON THEIR TRAVEL FROM THE SPINNERET TO SAID WATER WASHING ZONE AT A RATE OF AT LEAST 400 PERCENT OF THE RATE OF EXTRUSION OF SAID SOLUTION THROUGH SAID SPINNERET. 